The degrees of freedom of an image formed by any real instrument are only a finite number, while those those of the object are an infinite number. Several different objects may correspond to the same image.
It is shown that in the case of coherent illumination a large class of objects corresponding to a given image can be found very easily. Two-point resolution is impossible unless the observer has a priori an infinite amount of information about the object.
G. TORALDO DI FRANCIA, "Resolving Power and Information," J. Opt. Soc. Am. 45, 497-499 (1955)